The proposed investigation is a study of the development of thymic lymphocytes from hematopoietic stem cells. A population of cells enriched in prothymocytes will be prepared by isopycnic centrifugation, differential killing and differential adhesiveness. These cells will be assayed by their ability to repopulate the thymus of irradiated mice and by their response to the purified thymic hormone thymopoietin. The state of differentiation of the prothymocyte and the degree of maturation induced by thymopoietin will be evaluated and the relationship of these cells to the progenitors of B lymphocytes determined.